Microbes that live in people's bodies make up 90% of living cells and when disrupted could be a major cause of obesity, according to Tim Spector, a professor of genetic epidemiology at King's College London.

In a new book called The Diet Myth, he researched the links between microbes, food and health in an attempt to an gain insight into the burgeoning obesity crisis.

Meanwhile, a new report by London's Overseas Development Institute (ODI) found prices of fruit and vegetables had risen 30% in the UK since 1980 while those of processed foods, meat and dairy had dropped 25%. Ice cream experienced the biggest price fall.

Prof Spector's research also found sweeteners in diet fizzy drinks had adverse affects on metabolism, leading to weight gain, while fasting diets such as the 5:2 diet could benefit microbes and metabolism.

But he found some elements were down to genetics, with a third of people naturally possessing microbes which prevented them getting fat, while genes even determined some people's preference for salads, broccoli or garlic.

A statement from the publishers Weidenfeld and Nicolson, said: "Compared to our recent ancestors who lived outside cities, with rich and varied diets and without antibiotics, we have only a fraction of the diversity of species of microbes living in our guts. Scientists are only now starting to understand the long-lasting impact this has on all of us.

"Only by understanding what makes our own personal microbes tick and interact with our bodies can we overcome the confusion of modern diets and nutrition to regain the correct balance of our ancestors."

Prof Spector also tweeted: "Burger diet reduces microbe diversity by 40% in a few days."

And on using his son as a research subject for the book, he said: "Feeding junk food to my son's poor gut microbes - a massacre in the name of science."

In its report, entitled The Rising Cost of a Healthy Diet, the ODI analysed data from five countries, including the UK and USA.

The researchers said: "Typical UK diets are not balanced in accordance with dietary recommendations, with excessive consumption of grains and other starchy foods, protein-rich foods, oils, fats, and sugar - coupled with particularly low intake of fruit and vegetables."

And they suggested there may be a case for taxation of certain foods, although it would come down to "political appetite".

They said: "In terms of what might be taxed and subsidised, this report suggests that energy-dense foods might be taxed, while fruit and vegetables whose prices often rise compared to other foods, might be subsidised."